The strengthening of a steel sheet has been desired with the growing demands for weight reduction in automobiles. The strengthening of a steel sheet makes it possible to reduce the weight of an automobile through the reduction of material thickness and also to improve collision safety. Attempts have been made recently to form a material steel sheet or pipe of a high strength steel into components of complicated shapes by the hydroforming method for the purpose of reducing the number of components or welded flanges, in response to the demands for the weight reduction and cost reduction of an automobile. Actual application of new forming technologies, such as the hydroforming method (see Japanese Unexamined Patent Publication No. H10-175027), is expected to bring about great advantages such as the reduction of costs and increase in the degree of freedom in design work.
In order to fully enjoy the advantages of the hydroforming method, new materials suitable for the new forming methods are required. For instance, the influence of r-value on the hydroforming work was disclosed at the 50th Japanese Joint Conference for the Technology of Plasticity (in 1999, p. 447 of its proceedings). What was disclosed was, however, that, based on an analysis by a simulation, the r-value in the longitudinal direction was effective for T-shape forming, which was one of the fundamental forming modes of hydroforming. Apart from the above, as reported at FISITA World Automotive Congress, 2000A420 (Jun. 12-15, 2000, at Seoul), a high formability steel pipe was being developed aiming at realizing high strength and high ductility by forming fine crystal grains. The improvement of the r-value in the longitudinal direction of a steel pipe was also discussed in the report.
However, while the formation of fine crystal grains is very effective for securing ductility of thick materials, considering the points that, according to the report, fine crystal grains are obtained by warm working at comparatively low temperatures and that a heavy draft (the ratio of diameter reduction or area reduction, in this case) is applied during the working, it is possible that the reported method lowers the n-value, which is important for the forming by hydroforming and similar methods, and does not increase average r-value, which is an indicator of formability.
As reviewed above, there are very few cases of practical developments of materials suitable not only for a certain basic forming mode such as hydroforming or the like but also for various forming modes. Thus, in the absence of suitable materials, conventional high r-value steel sheets and high ductility steel sheets are used for the hydroforming applications.